1. Field of the Invention
The invention relates to forming metal leads, and more particularly to flexible techniques for excising and forming metal leads bonded to an electronic device which extend over a sidewall of the device.
2. Description of Related Art
In the manufacture of integrated circuits, silicon wafers containing many individual integrated circuits are fabricated and then the wafers are cut into individual integrated circuit chips or die. The chips are subsequently packaged and interconnected to other electrical components. During packaging, electrically conductive metal leads can be bonded between a chip and an interconnect substrate to provide proper electrical interconnection between different chips.
One technique for providing conductive interconnecting metal leads is tape-automated-bonding (TAB). The leads can be fabricated on a continuous carrier film in which the tape is a laminate of copper and a plastic insulator and the leads are etched in the copper while on the plastic carrier using well known etching processes. Or an additive metal fabrication can be used. The film is normally perforated along the edges with sprocket holes for use in advancement and alignment of the carrier film. Individual chips can be electrically bonded to individual lead frames in the tape, for instance by laser bonding as described in U.S. Pat. No. 4,845,335, and the lead frames can be removed or excised from the carrier film. Usually it is necessary to bend or form the leads prior to bonding the outer lead ends to an interconnect substrate.
Conventional excise and lead form of TAB devices requires the design and fabrication of custom tooling for each device size and/or lead form geometry. These tools basically conform to punch and die technology where the device is forced through a cutting die to excise it from a TAB film strip, and then forced around a forming die to produce the lead form. See, for instance, U.S. Pat. Nos. 2,997,907; 3,417,643; 4,064,917; 4,361,173; 4,371,012; 4,488,581: 4,559,978: 4,625,772; 4,633,920: 4,691,747; 4,787,426; 4,945,954 and 5,065,504.
As the pitch for TAB devices decreases (8 mils or less) it becomes more difficult to maintain the outer lead pitch during the excise and lead form operation. During excise sequence the device is literally punched through a cutting die. The cutting allowance is either designed in with a one-piece cutting die or adjustable with a four-piece cutting blade matrix. If this cutting allowance is incorrect, burrs will form and often cause lead splay.
Once excised from the TAB film strip, the leads become cantilevered beams supported by the inner lead bond and often a dielectric support ring. These lead beams have a very small cross-sectional area as compared to their length, making them susceptible to bending. As these beams are forced around a forming die, they are bent closest to the supported end first and continually abraded down their length until the second or lower bend is formed, making it difficult to control the pitch at the other end. Keeper bars (strips of dielectric on the lead tips) are often used to prevent splay on fine pitch devices; however, they may require more complex form tooling and present other problems during outer lead bonding.
In addition, due to the tight tolerances required on fine pitch devices, the punches and dies must be customized for the exacting specifications of each TAB design. As a result, the tools are expensive, require long lead times to procure, are labor intensive and are time consuming to install and tune. Once on line, this is an effective and relatively fast (e.g., 6 to 8 seconds) method of excise and lead form. However, less static activities such as the research and development of TAB multichip modules may be hindered.
Needless to say, there is a need for an alternative approach to TAB excise and lead form that allows an inexpensive set of hard tooling to be used for a variety of chip sizes and beam tape geometries while minimizing splaying and galling of the leads.
SUMMARY OF THE INVENTION
An object of the present invention is to provide excise and lead form of TAB devices with varying sizes and lead geometries without the need to change hand tooling. The present invention requires only a change in programming to accommodate a wide variety of devices.
A feature of the present invention includes a method of excising and forming a plurality of metal leads connected at inner ends to an electronic device which extend beyond a sidewall of the device, comprising the steps of clamping a first portion of the leads spaced from the device by contacting the top of the first portion with the bottom surface of a first lead clamp and the bottom of the first portion with the top surface of a form anvil, said form anvil further comprising a forming edge between the top surface and a forming surface on the side opposite the device, clamping a second portion of the leads further from the device than the first portion and spaced from the first portion by contacting the top of the second portion with the bottom surface of an excise/form tool and contacting the bottom of the second portion with the top surface of a second lead clamp, said excise/form tool further comprising a forming surface on the side facing the device and an outer surface on the side opposite the device and a forming edge between the bottom surface and the forming surface, and said second lead clamp further comprising an outer surface on the side opposite the device, wherein the outer surfaces of the excise/form tool and second lead clamp are aligned, excising a third portion of the leads further from the device than the second portion which extends beyond the outer surfaces of the excise/form tool and second lead clamp, thereby forming outer lead ends, moving the excise/form tool and second lead clamp downwards and toward the device until a fourth portion of the leads between the first and second portions is moved through an arc between the forming surfaces of the form anvil and the excise/form tool, thereby bending the leads at the forming edge of the form anvil to form a first corner in the leads and at the forming edge of the excise/form tool to form a second corner in the leads, and releasing the leads from the first lead clamp, form anvil, form/excise tool, and second lead clamp.
An advantage of the present invention is the use of one set of low cost tooling which can rapidly adapt to a variety of device sizes and lead form geometries.
Another advantage of the present invention is that the leads are not forced around a forming die, but rather are clamped and bent at two programmable points before excise and during lead form. This eliminates galling of the vertical leg and minimizes splaying at the outer lead ends.
A still further advantage of the present invention is that the vertical leg of the leads may contain a dielectric support bar, thereby decreasing the footprint of the device and allowing closer chip-to-chip spacing.
Still another advantage of the present invention is the ability to perform face-up and face-down lead forms with the same tooling.
These and other objects, features and advantages of the present invention will be further described and more readily apparent from a review of the detailed description and preferred embodiments which follow.